Method and apparatus to provide low cost transmit beamforming for network devices

ABSTRACT

Techniques and structures for use in generating an approximated beamforming matrix in a MIMO based system are disclosed. The techniques and structures may be used to allow closed loop MIMO beamforming to be performed within a device that does not include singular value decomposition (SVD) circuitry.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/076,195, entitled “METHOD AND APPARATUS TO PROVIDE LOW COST TRANSMITBEAMFORMING FOR NETWORK DEVICES,” filed Mar. 09, 2005 (attorney docketno. P20439).

TECHNICAL FIELD

The invention relates generally to wireless communication and, moreparticularly, to techniques and structures for implementing closed loopMIMO in a wireless network.

BACKGROUND OF THE INVENTION

Multiple input multiple output (MIMO) is a radio communication techniquein which both a transmitter and a receiver use multiple antennas towirelessly communicate with one another. By using multiple antennas atthe transmitter and receiver, the spatial dimension may be takenadvantage of in a manner that improves overall performance of thewireless link. MIMO may be performed as either an open loop or a closedloop technique. In open loop MIMO, the transmitter has no specificknowledge of the condition of the channel before data signals aretransmitted to the receiver. In closed loop MIMO, on the other hand, thetransmitter uses channel-related information to precondition transmitsignals before they are transmitted to better match the present channelstate. In this manner, performance may be improved and/or receiverprocessing may be simplified. There is a need for techniques andstructures for efficiently implementing closed loop MIMO in wirelessnetworks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example wireless networkingarrangement in accordance with an embodiment of the present invention;

FIG. 2 is a block diagram illustrating an example processing arrangementin accordance with an embodiment of the present invention;

FIG. 3 is a signal diagram illustrating an example continuous frameexchange sequence that may be used within a wireless network inaccordance with an embodiment of the present invention;

FIG. 4 is a flowchart illustrating an example method for use insupporting closed loop MIMO operation in a wireless network inaccordance with an embodiment of the present invention;

FIG. 5 is a flowchart illustrating an example method for use ingenerating an approximated beamforming matrix using a linear filter inaccordance with an embodiment of the present invention; and

FIG. 6 is a flowchart illustrating an example method for use during acontinuous frame exchange in a wireless network in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the invention may be practiced. These embodimentsare described insufficient detail to enable those skilled in the art topractice the invention. It is to be understood that the variousembodiments of the invention, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described herein in connection with one embodiment may beimplemented within other embodiments without departing from the spiritand scope of the invention. In addition, it is to be understood that thelocation or arrangement of individual elements within each disclosedembodiment may be modified without departing from the spirit and scopeof the invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined only by the appended claims, appropriately interpreted, alongwith the full range of equivalents to which the claims are entitled. Inthe drawings, like numerals refer to the same or similar functionalitythroughout the several views.

FIG. 1 is a block diagram illustrating an example wireless networkingarrangement 10 in accordance with an embodiment of the presentinvention. As illustrated, a wireless access point (AP) 12 iscommunicating with a wireless station (STA) 14 via a wirelesscommunication link. The wireless AP 12 may be providing access to alarger network (wired and/or wireless) for the STA 14. The STA 14 mayinclude any type of wireless component, device, or system that iscapable of accessing a network through a remote wireless access point.Although only a single STA is shown in FIG. 1, it should be appreciatedthat the wireless AP 12 may be capable of providing access services tomultiple STAs simultaneously. As illustrated, the wireless AP 12 and theSTA 14 each have multiple (i.e., two or more) antennas. Any type ofantennas may be used including, for example, dipoles, patches, helicalantennas, and/or others. The wireless channel between the AP 12 and theSTA 14 is a multiple input, multiple output (MIMO) channel.

In the embodiment of FIG. 1, the wireless AP 12 includes a wirelesstransceiver 16 and a controller 18. The controller 18 is operative forcarrying out the digital processing functions required to support closedloop MIMO operation for the AP. The controller functions may be carriedout using, for example, one or more digital processing devices such as,for example, a general purpose microprocessor, a digital signalprocessor (DSP), a reduced instruction set computer (RISC), a complexinstruction set computer (CISC), a field programmable gate array (FPGA),an application specific integrated circuit (ASIC), and/or others,including combinations of the above. The controller 18 may also includeone or more discrete digital elements such as, for example, bitinterleavers, bit de-interleavers, modulation units, demodulation units,discrete Fourier transform units, inverse discrete Fourier transformunits, etc. The wireless transceiver 16 is operative for performing theradio frequency (RF) related functions required to (a) generate RFtransmit signals for delivery to the multiple antennas during transmitoperations and (b) process the RF signals received by the multipleantennas during receive operations. Separate transmit and receive chainsmay be provided within the transceiver 16 for each correspondingantenna. Digital to analog converters and analog to digital convertersmay be used in the interface between the controller 18 and thetransceiver 16. The STA 14 of FIG. 1 also includes a wirelesstransceiver 20 and a controller 22. These elements may perform functionssimilar to the corresponding units within the AP 12 (although the APwill typically be capable of supporting multiple simultaneous wirelessconnections while the STA may only be capable on supporting one).

In at least one embodiment, the AP 12 and the STA 14 may be capable ofoperation using orthogonal frequency division multiplexing (OFDM)techniques. In an OFDM system, data to be transmitted is distributedamong a plurality of substantially orthogonal, narrowband subcarriers.The AP 12 and/or the STA 14 may also be capable of operation using aform of MIMO known as SVD (i.e., singular value decomposition) MIMO. SVDMIMO will be discussed in greater detail below. To facilitateunderstanding and simplify notation, the discussion that follows may bewith respect to a single subcarrier in an OFDM system. It should beappreciated that the below described functions may need to be performedfor each of the subcarriers within a multi-carrier system. Interpolationbetween subcarriers may also be used to reduce the amount ofcalculation.

In a MIMO-based system, a wireless channel may be characterized using ann_(RX)×n_(TX) channel matrix H, where n_(RX) is the number of receiveantennas and n_(TX) is the number of transmit antennas. Using SVD, thechannel matrix H may be decomposed as follows:

H=UDV^(H)   (Equation 1)

where U and V are unitary matrices (i.e., matrices with orthonormalcolumns and unit amplitude), D is a diagonal matrix, and V^(H) is theHermitian of unitary matrix V. A unitary matrix Q has the followingproperty:

Q^(H)Q=I

where I is the identity matrix. In the channel matrix decomposition setout above, the matrix V may be referred to as the beamforming matrix(precoder). This beamforming matrix V may be generated by firstdetermining the channel matrix H for the MIMO channel and thendecomposing the matrix H using SVD techniques (or other similartechniques). The beamforming matrix V may then be used to process asubsequent transmit signal to be transmitted through the MIMO channel. Aseparate matrix V may be required for each subcarrier in a multicarriersystem.

The elements of the diagonal matrix D are known as the singular values,or eigenvalues, of the channel matrix H. The beamforming matrix V ismade up of a number of column vectors, known as eigenvectors, thatcorrespond to the eigenvalues. Each of the eigenvectors may define aspatial channel (or eigenmode) within the MIMO channel. The stream ofdata flowing through a particular spatial channel is known as a spatialstream. The eigenvalues will typically be indicative of the relativestrength of the corresponding eigenvectors/spatial channels. Sometimes,it may be advantageous to limit a MIMO transmission to only thestrongest of the available spatial channels (e.g., to the spatialchannels associated with the 2 largest eigenvalues).

In at least one embodiment of the present invention, a closed loop MIMOchannel is provided that utilizes implicit feedback techniques. Implicitfeedback relies on the property of channel reciprocity to obtaininformation about the MIMO channel within a transmitting device.Implicit feedback requires calibrations to be performed for thetransmitting device and the receiving device to accurately model theoverall channel as a reciprocal component. After calibrations have beenaccomplished, training signals may be transmitted from the receivingdevice to the transmitting device to allow the transmitting device tocalculate channel information for the reverse channel. The reciprocalproperty of the channel may then be used to determine the channelinformation in the forward direction from the transmitting device to thereceiving device.

With reference to FIG. 1, the wireless downlink channel from theantennas of the AP 12 to the antennas of the STA 14 may be characterizedusing a channel matrix H. In the uplink direction, from the antennas ofthe STA 14 to the antennas of the AP 12, the wireless channel may becharacterized as the transpose of the channel matrix H (i.e., H^(T)),based on channel reciprocity. In each direction, the overall channelwill also include components from within the devices themselves (i.e.,the AP 12 and the STA 14). For example, in the downlink direction, theoverall channel may be expressed as:

H^(d)=β_(STA)Hα_(AP)

where α_(AP) is a component characterizing a transmitter portion of theAP, H is the channel matrix from the transmit antennas to the receiveantennas, and β_(STA) is a component characterizing a receiver portionof the STA. Likewise, in the uplink direction, the overall channel maybe expressed as:

H^(u)=β_(AP)H^(T)α_(STA)

where α_(STA) is a component characterizing a transmitter portion of theSTA, H^(T) is the channel matrix from the transmit antennas to thereceive antennas in the uplink direction, and β_(AP) is a componentcharacterizing a receiver portion of the AP. The calibrations discussedabove for implicit feedback systems may be performed to determine valuesfor α_(AP), β_(STA), α_(STA), and β_(AP). Once these parameters havebeen determined, the overall downlink channel may be determined by: (a)transmitting training data from the STA 14 to the AP 12, (b) using thetraining data within the AP 12 to determine the overall wireless channelmatrix H^(u)=β_(AP)H^(T)α_(STA), (c) performing a transpose operation onH^(u), (d) using α_(STA) and β_(AP) to generate the wireless channelmatrix H (H=β_(AP) ⁻¹H^(u)α_(STA) ⁻¹), and (e) using β_(STA) and α_(AP)to generate the overall downlink channel (H^(d)=β_(STA)Hα_(AP)). Inanother embodiment, circuit compensations may be conducted to remove theeffect of α_(AP), β_(STA), α_(STA), and β_(AP). Namely, thecompensations set the matrixes to be scaled identity matrixes. In thiscase, the channel matrix up to a global scaling factor can be directlyobtained from the received channel training data (with only transposeoperation) without additional processing related to α_(AP), β_(STA),α_(STA), and β_(AP). After a channel matrix has been determined for theoverall downlink channel, a beamforming matrix V may be determined byperforming an SVD operation on the channel matrix. The beamformingmatrix V may then be used to develop transmit signals for delivery intothe MIMO channel from the AP 12. A similar process may be used todetermine the channel matrix and beamforming matrix for the uplinkchannel.

In the procedure outlined above, an SVD operation is performed on thechannel matrix to determine the beamforming matrix V for use ingenerating transmit signals. The SVD operation is often performed byspecialized SVD circuitry and usually requires several iterations tocomplete. The presence of SVD circuitry increases the silicon cost of animplementing system and also increases overall power consumption. Thecomputational latency of the SVD circuits may be reduced by increasingthe gate count of the circuitry, but this further increases the siliconcost. Because of the cost, power consumption, and/or computationallatency of SVD circuits, it may be undesirable to include suchstructures within certain types of network devices (e.g., STA devices,etc.). In at least one aspect of the present invention, a lower costalternative is provided for determining a beamforming matrix within awireless device, component, or system.

In the IEEE 802.11n high throughput wireless networking standard that iscurrently in development, it is believed that open loop MIMO capabilitywill be mandatory. As described previously, in a system implementingopen loop MIMO, a transmitting device generally has no specificknowledge of the condition of the channel before transmitting datasignals to a receiving device. The receiving device will typicallyinclude a linear filter, such as a minimum mean square error (MMSE)filter or a zero-forcing filter, to process the received signal torecover the transmitted data. In conceiving the present invention, itwas appreciated that the linear filter that is present within a deviceto perform open loop MIMO receive functions may be leveraged for useduring closed loop MIMO operation to determine an estimate orapproximation of a beamforming matrix V for use by the device during asubsequent transmit operation. In this manner, a STA (or other networkdevice, component, or system) that does not include SVD circuitry maystill be able to perform closed loop MIMO transmit beamforming by takingadvantage of circuitry that is already available for use in open loopMIMO operation.

FIG. 2 is a block diagram illustrating an example processing arrangement30 in accordance with an embodiment of the present invention. Theprocessing arrangement 30 may be implemented within any type of networkdevice including, for example, STAs, APs, and others. In the discussionthat follows, it will be assumed that the processing arrangement 30 iswithin a STA. As illustrated, the processing arrangement 30 includes: alinear filter 32, a beamforming matrix estimator 34, and a transmitsubsystem 36. A group of local antennas receives signals from a remoteAP through a MIMO channel. The linear filter 32 processes these receivedsignals to generate an output matrix. The linear filter 32 may include,for example, an MMSE or zero-forcing filter. The output of the linearfilter 32 will typically need to undergo further receive processing toextract the useful data from the received signals. During closed loopMIMO operation, the output matrix of the linear filter 32 may also bedirected to the beamforming matrix estimator 34 for use in developing anapproximated beamforming matrix for use in the uplink channel. Theapproximated beamforming matrix may then be delivered to a transmitsubsystem 36 for use in developing transmit signals for transmission tothe AP via the uplink channel.

As illustrated in FIG. 2, in at least one embodiment, the beamformingmatrix estimator 34 includes: a row orthogonalizer 38, a transposefunction 40, and a conjugation function 42. During closed loopoperation, the remote AP may multiply a vector of data symbols d by abeamforming matrix V to generate a transmit signal vector x fortransmission from multiple antennas (i.e., x=Vd). The transmit vector xwill then be acted upon by the channel H and received by the antennasassociated with processing arrangement 30. The received signal (i.e.,HVd) is directed to the linear filter 32 for processing. For azero-forcing receiver, the linear filter 32 computes the inverse of thecombined channel HV to generate output matrix W=(HV)⁻¹. From Equation 1,the matrix W may be expressed as follows:

W=(HV)⁻¹=(UD)⁻¹=D⁻¹U′

where U′ is the conjugate transpose of matrix U. For an MMSE receiver,the linear filter 32 computes:

$W = {{\lbrack {{({HV})^{\prime}({HV})} + {\sigma^{2}I}} \rbrack^{- 1}({HV})^{\prime}} = {\underset{P}{\underset{}{\lbrack {{D^{\prime}D} + {\sigma^{2}I}} \rbrack^{- 1}D^{\prime}}}{U^{\prime}.}}}$

where ' denotes the operation of conjugate transpose and P is a diagonalmatrix. Seen from the two equations above, the output matrix W is theproduct of a diagonal matrix on the left and a unitary matrix U′ on theright for both zero-forcing and MMSE receivers. As H is the channelmatrix for the downlink channel, the channel matrix for the uplinkchannel is H^(T) for per antenna training and (HV)^(T) for per streamtraining. For both of these cases, the beamforming matrix of the STAwill be the same. The following discussion will focus on per antennatraining. It is assumed that the implicit feedback calibrations andcompensations discussed above have been performed for both the STA andthe AP. The SVD of the channel matrix in the reverse direction (H^(T))is as follows:

H^(T)=V*DU^(T).

Thus, the beamforming matrix of the STA is the conjugate of matrix U(i.e., U*). The row orthogonalizer 38, the transpose function 40, andthe conjugation function 42 may be used to estimate U* using the outputof the linear filter 32 as follows. The row orthogonalizer 38 firstorthogonalizes the rows of matrix W. Any orthogonalization technique maybe used including, for example, QR decomposition, Gram-Schmidtorthogonalization, and/or others. The output of row orthogonalizer 38 isa unitary matrix having rows that are orthogonal to each other and thateach have a unity norm. For the case where the exact V matrix is appliedby the AP, the rows of W computed by the STA are already orthogonal toeach other for both zero-forcing and MMSE receivers. In this case, onlynormalization of the rows of W is needed in the row orthogonalizer 38.The transpose function 40 takes the transpose of the orthogonalizedmatrix W to approximate the matrix U. The conjugate function 42 may thenbe used to approximate the conjugate of matrix U (i.e., U*), which isthe beamforming matrix to be used by the STA. For the case where V isapplied by the AP and a zero-forcing (or MMSE) receiver is employed bythe STA, the approximation is exact. As described above, this estimatemay be delivered to the transmit subsystem 36 for use in developingtransmit signals to be transmitted to the AP via the local antennas. Itshould be appreciated that the physical order of the row orthogonalizer38, the transpose function 40, and the conjugation function 42 in FIG. 2may be changed while still achieving the desired result.

In at least one scenario, a wireless AP in a network will be outfittedwith SVD circuitry and a STA in the network will not have SVD circuitry.The STA may thus use the above-described technique to perform closedloop beamforming in the uplink direction. The AP, on the other hand, mayalso make use of the estimation technique, even if it has SVD circuitryavailable. This may be done to, for example, reduce the overallcomputation complexity and/or energy consumption within the AP. The useof an approximation may also improve computational latency in the AP.FIG. 3 is a signal diagram illustrating an example continuous frameexchange sequence 50 that may be used within a wireless network inaccordance with an embodiment of the present invention. The frameexchange is between a wireless AP and a STA in the network. The upperportion of the diagram represents the transmissions of the wireless APand the lower portion represents the transmissions of the STA. Thewireless AP and the STA both include multiple antennas. The wireless APhas SVD circuitry on-board. As illustrated, the AP transmits a firstframe 52 to the STA using an exact beamforming matrix. Although notshown, a training exchange may have preceded the first frame 52 of theframe exchange sequence 50, during which the exact beamforming matrix Vwas generated. As used herein, the term “exact” beamforming matrix meansa beamforming matrix that is generated based on actual channel trainingusing SVD circuitry, as opposed to an approximated beamforming matrix asdescribed above.

After receiving the first frame 52, the STA may transmit a second frame54 using an approximated beamforming matrix. The approximatedbeamforming matrix may be generated as described above using a zeroforcing filter, an MMSE filter, or some other form of linear filterhaving the requisite properties. The second frame 54 may be, forexample, an acknowledgement (ACK) frame. In at least one embodiment, thesecond frame 54 may also include, among other things, reverse directionuser data. After receiving the second frame 54, the AP may transmit athird frame 56 using an approximated beamforming matrix. That is,instead of generating another “exact” beamforming matrix, the AP may usean on-board zero forcing filter or MMSE filter to generate anapproximated beamforming matrix, as described above, for use intransmitting the third frame 56. The STA and the AP may then both useapproximated beamforming matrices for the remainder of the frameexchange sequence 50 (e.g., fourth frame 58, fifth frame 60, and so on).As long as the channel is not changing too rapidly, the approximatedbeamforming matrices generated by both the AP and the STA should remainrelatively accurate. In another approach, the AP may always use an exactbeamforming matrix during the frame exchange sequence 50, generatedusing the on-board SVD circuitry.

The approximation of the ideal beamforming matrix can be checked by thereceiver that receives the beamformed signals. For example, in the aboveexample, the AP can check the crosstalk (or orthogonality) between rowsof the matrix W that the AP computes. If the AP finds that the crosstalkexceeds a certain level, then the approximation (of the beamformingmatrix) of the STA may not be accurate enough. This may be due to thefact that the approximation at the AP is not accurate. The AP canactivate its SVD circuit to compute the exact beamforming matrix for thedownlink. This will bring back the accuracy for both downlink anduplink.

In general, if one side of a link employs an exact beamforming matrixand the other side employs a zero-forcing or MMSE receiver as describedabove to generate an approximation, the approximation on the receiverside will be close to exact. Because the approximation is close toexact, the beamforming in the return link also employs the exactbeamforming matrix. The accuracy of beamforming should continue duringthe exchange sequence if the channel remains the same. If the channelchanges or the channel estimation is corrupted by noise, theapproximation of the beamforming matrix will not be exact. In at leastone embodiment of the present invention, the side of the link with theSVD circuitry may occasionally check the accuracy of the approximationby checking the crosstalk between the rows of W, and recover theaccuracy with an exact beamforming matrix in the return link byactivating the SVD circuit.

FIG. 4 is a flowchart illustrating an example method 70 for use insupporting closed loop MIMO operation in a wireless network inaccordance with an embodiment of the present invention. The method 70may be practiced in connection with, for example, a STA or AP within awireless network. First, a signal is received from a wireless MIMOchannel at multiple antennas (block 72). A linear filter is then used toprocess the received signal to facilitate the approximation of abeamforming matrix for use in the return channel (block 74). The linearfilter may include, for example, a zero-forcing filter, an MMSE filter,and/or others. The approximated beamforming matrix may then be used togenerate transmit signals for transmission in the return MIMO channel(block 76).

FIG. 5 is a flowchart illustrating an example method 80 for use ingenerating an approximated beamforming matrix using a linear filter inan embodiment of the present invention. The method 80 may be used, forexample, as part of the method 70 of FIG. 4. First, a received signal isprocessed in a linear filter to generate a matrix W (block 82). The rowsof the matrix W are then orthogonalized (block 84). Anyorthogonalization technique may be used including, for example, QRdecomposition, Gram-Schmidt orthogonalization, and/or others. Thetranspose of the orthogonalized matrix W is then determined to generatematrix U (block 86). The complex conjugate of the matrix U is thendetermined to form the approximated beamforming matrix for the returnchannel (block 88).

FIG. 6 is a flowchart illustrating an example method 90 for use during acontinuous frame exchange in a wireless network in accordance with anembodiment of the present invention. An exact beamforming matrix isfirst acquired by an AP for a downlink MIMO channel (block 92). Theexact beamforming matrix may be acquired by, for example, a previoustraining operation. The exact beamforming matrix is then used togenerate a transmit frame for delivery to a STA (block 94). A responseframe is then received by the AP from the STA (block 96). A linearfilter, such as a zero-forcing filter or an MMSE filter, is then used togenerate an approximated beamforming matrix for the downlink channel(block 98). The approximated beamforming matrix is then used to generateanother transmit frame for delivery to the STA (block 100). Thebeamforming matrix approximation may then be performed for eachsubsequent response frame received from the STA until the continuousframe exchange terminates.

The techniques and structures of the present invention may beimplemented in any of a variety of different forms. For example,features of the invention may be embodied within laptop, palmtop,desktop, and tablet computers having wireless capability; personaldigital assistants having wireless capability; cellular telephones andother handheld wireless communicators; pagers; satellite communicators;cameras having wireless capability; audio/video devices having wirelesscapability; network interface cards (NICs) and other network interfacestructures; integrated circuits; as instructions and/or data structuresstored on machine readable media; and/or in other formats. Examples ofdifferent types of machine readable media that may be used includefloppy diskettes, hard disks, optical disks, compact disc read onlymemories (CD-ROMs), magneto-optical disks, read only memories (ROMs),random access memories (RAMs), erasable programmable ROMs (EPROMs),electrically erasable programmable ROMs (EEPROMs), magnetic or opticalcards, flash memory, and/or other types of media suitable for storingelectronic instructions or data. In at least one form, the invention isembodied as a set of instructions that are modulated onto a carrier wavefor transmission over a transmission medium.

It should be appreciated that the individual blocks illustrated in theblock diagrams herein may be functional in nature and do not necessarilycorrespond to discrete hardware elements. For example, in at least oneembodiment, two or more of the blocks in a block diagram are implementedwithin a single digital processing device. The digital processing devicemay include, for example, a general purpose microprocessor, a digitalsignal processor (DSP), a reduced instruction set computer (RISC), acomplex instruction set computer (CISC), a field programmable gate array(FPGA), an application specific integrated circuit (ASIC), and/orothers, including combinations of the above. Hardware, software,firmware, and hybrid implementations may be used.

In the foregoing detailed description, various features of the inventionare grouped together in one or more individual embodiments for thepurpose of streamlining the disclosure. This method of disclosure is notto be interpreted as reflecting an intention that the claimed inventionrequires more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive aspects may lie in less thanall features of each disclosed embodiment.

Although the present invention has been described in conjunction withcertain embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art readily understand.Such modifications and variations are considered to be within thepurview and scope of the invention and the appended claims.

1. A method of performing implicit beamforming by a transmitting stationin a wireless network, comprising: transmitting a first training signalto a receiving station using a current beamforming vector; receiving asecond training signal from the receiving station; selecting a newbeamforming vector as the current beamforming vector based on thereceived second training signal; and repeating said transmitting,receiving, and selecting until a condition is met.
 2. The method ofclaim 1, wherein repeating repeats said transmitting, receiving, andselecting at least three times.
 3. The method of claim 1, wherein thecondition is a termination event.
 4. The method of claim 3, wherein thetermination event is a termination of exchanging frames.
 5. The methodof claim 1, further comprising: transmitting data using the currentbeamforming vector subsequent to the condition being met.
 6. A method ofperforming implicit beamforming by a receiving station in a wirelessnetwork, comprising: receiving a first training signal from atransmitting station; computing a current beamforming vector based onthe first transmitting signal; transmitting a second training signal tothe transmitting station using the current beamforming vector; andrepeating said receiving, computing, and transmitting until a conditionis met.
 7. The method of claim 6, wherein repeating repeats saidreceiving, computing, and transmitting at least three times.
 8. Themethod of claim 6, wherein the condition is a termination event.
 9. Themethod of claim 8, wherein the termination event is a termination ofexchanging frames or a termination of calibration.
 10. The method ofclaim 6, further comprising: receiving data using the currentbeamforming vector subsequent to the condition being met.
 11. A methodof performing implicit beamforming by a transmitting station in awireless network, comprising: transmitting a first training signal to areceiving station using a transmit beamforming vector; receiving asecond training signal from the receiving station; selecting a newtransmit beamforming vector as based on the received second trainingsignal; and repeating said transmitting, receiving, and selecting untilexchanging frames is terminated or until calibration is terminated. 12.The method of claim 11, wherein repeating repeats said transmitting,receiving, and selecting at least three times.
 13. The method of claim11, further comprising: transmitting data using the transmit beamformingvector after the calibration is terminated.